The construction of automotive bodies and other manufacturing operations often requires the formation of many uniformly sized electrical resistance spot welds. The welds (or weld nuggets) are made by bringing one or more assemblies of sheet metal parts to a welding station where a computer controlled robot, or other welding machine(s), systematically and sequentially forms a series of spot welds to attach the parts of the assembly or assemblies into integral bodies.
The welding is performed using a weld gun typically having copper or copper alloy welding electrodes (i.e., a fixed electrode and a movable electrode). The electrodes are axially aligned in opposition to each other. They are positioned around a workpiece at a specified spot weld site and the movable electrode is advanced to clamp and press the workpiece against the fixed electrode. A large electric current is momentarily passed between the opposing electrodes through the electrically resistive metal pressed between them. The sheet metal between the electrodes is briefly fused during current flow and then re-solidified to form an integral weld nugget of suitable diameter at the faying surfaces of the sheet metal layers. In such a manufacturing operation, scores or hundreds of such welds are rapidly formed. The goal is to form all of the welds to substantially the same size, within an acceptable tolerance value, and with minimal internal porosity or discontinuities.
Welding controllers are used to control the force applied by the movable electrode on the metal surface at the weld site. The controller also is programmed to control the weld current and its duration. The force applied to the welding site by the electrode and the resistive heat generated by the welding current results in an indentation (caused by the displacement of the electrode) in the softened welded surface of the workpiece at the location of the weld nugget. In setting up the welding machine to produce a series of uniform welds, initial values of suitable electrode force, total welding current, and its duration are established for the welding gun and the specific workpieces. The welding controller can be programmed in an attempt to maintain these values so that the same welds are produced during extended manufacturing operations. But variations creep into the operation because of electrode wear and workpiece gap and orientation. Furthermore, there may also be variations in the operation of the welding machine(s).
Weld nugget uniformity and quality can be confirmed off-line from the production operation by periodically removing the workpieces from the operation and physically examining and testing a number of weld nuggets. But it would be preferable to have an on-line process that could be performed during welding operations using a computer-based weld controller to assess welding conditions and correlate them, if possible, to weld nugget quality and size.
Thus, it is an object of the present invention to provide a method of controlling on-going welding operations as a sequence of spot welds are formed to compensate for variations in the process so as to reduce variations in the size and integrity of the individual weld nuggets. It is a further object of the present invention to provide a control method that can be performed by a suitable weld controller and can easily be implemented without stopping the welding operation.